Nanoporous anodic oxide ceramic membranes such as aluminum oxide, titanium oxide, etc. are of interest for various applications, such as filters, gas sensors, catalysis, hemodialysis, electro dialysis, fuel cells, templates for nanostructures, life science and biomedical applications, and so on. Different from other ceramic membranes, they have through-hole pores of uniform size, so that they can effectively and efficiently filter harmful particles smaller than the pore sizes without significant fouling, which is one of the serious problems which occurs when using conventional porous membranes with tortuous pore channels of wide distributions in size, and thus can be used for much longer than conventional ones without frequent cleaning the pore channel. Furthermore, their pore size can be controlled from several to several hundred nanometers, so that the selectivity in filtering of the filter incorporating the membranes is likely very excellent.
The pore size of nanoporous anodic aluminum oxide membranes can be controlled by selecting an electrolyte among sulfuric acid, phosphoric acid, oxalic acid, malonic acid, tartaric acid, citric acid and a mixture of sulfuric and oxalic acids, and applying the voltage specifically corresponding to the electrolyte. The pores can be self-ordered in an array of a close-packed structure by a two-step anodization process. Or the pore can be forced to be well-ordered by using imprints where nanoscale lattice patterns of convex features are present.
It has been known that as the melting point of the nanoporous anodic aluminum oxide is 1,000° C., it is estimated that they can be used up to 600° C. However, after heat treatment in high temperatures for phase conversion to crystalline forms, they can be used in high temperatures up to 1,000° C. The transformed ones are chemically very stable so that they can be used even in severe alkaline and acidic circumstances. That means that they can be used in ultrafiltration, nanofiltration and reverse osmosis in severely adverse acidic and alkaline environmental conditions at very high temperatures. Also nanoporous anodic aluminum oxide membranes are compatible with human organisms so that their application in life science including hemodialysis where fouling is a very severe problem, is of much attention. Furthermore, coating of layers being capable of adsorbing harmful gas, catalytic layers or layers with special functionality on pore walls and the surfaces of the membranes facilitates the membranes to be used as filters and sensors with very excellent performance. So the importance of nanoporous anodic aluminum oxide membranes in filter and sensor applications has become progressively large.
Nanoporous anodic titanium oxide membranes of different pore size can be made by using similar anodization methods with those for alumina ones, except that different electrolyte may be used. The electrolyte may comprise HF, KF, NaF, and a mixed solution of H2SO4 and HF, CrO3 and HF, (NH4)2SO4 and NH4F, and (NH4)2SO4 and NaF. The wall thickness of titanium oxide anodized is generally independent of the duration of the anodizing process.
As titanium oxide also shows excellent chemical resistance, it can be used in severe alkaline and acidic atmosphere. Its three crystalline phases, anatase, rutile and brookite, shows photo-catalytic activity so that nanoporous titanium oxide membranes can be used as filters decomposing environmentally harmful gases like VOC, NOx, SOx, etc., without depositing any other catalytic materials on its surfaces and pores. Furthermore, its high melting temperature of 1870° C. allows nanoporous titania membranes to be used in high temperatures. Therefore they can be used in severely acidic and alkaline environmental conditions in very high temperatures.
To date, only film-type flat anodic alumina or titania membrane plates of high quality have been fabricated. By the way, application of such plates is restricted to just a few fields, due to their relatively small filtering area. Most filter application requires the membranes of tube form which are of a larger filtering area and thus offer high filtering efficiency. Even hollow fiber membranes have been used for much higher efficiency, and they have been used even in hemodialysis which needs a short filtration time as possible. Such hollow fiber membranes, however, have been made only by using polymer-based materials. Up to now porous ceramic membrane tubes have been fabricated using conventional methods like sol-gel process based slip-casting. However, their pore characteristic is not good as in anodic oxide ceramic membranes. Accordingly, the advent of nanoporous anodic oxide ceramic membranes of tubular and hollow fiber shapes with excellent pore characteristic will remove all the disadvantages resulting from the flat and plate shape, and accelerate their practical applications to many fields. However, there have been no reports of making nanoporous anodic oxide ceramic membrane tubes with excellent pore characteristic.
High quality anodic oxide ceramic membranes with excellent pore characteristic are ones where close-packed through-hole pores of uniform size are well-arranged in an ordered manner and the porosity is high. For the fabrication of such high quality ones, by the way, special care must be paid for uniform electric field distribution between anode and cathode, excellent heat release capability from anode during anodization, uniform flow pattern of electrolyte, especially near the surface of metal to be anodized, and so on. Achievement of such requirements is relatively easy when plates are anodized, but not easy when metal tubes are anodized. So high quality anodic ceramic plates have been successfully fabricated by anodizing high purity aluminum or titanium plates using an anodization apparatus of relatively simple configuration, which satisfies the requirements mentioned above. However, it is likely that satisfaction of such requirements for tubes is not simple and easy. So few try has been made to fabricate anodic oxide ceramic membrane tubes.
The present invention shows that such requirements can be fulfilled in a relatively simple manner with fabrication methods incorporating cylindrical symmetry in arrangement of anode and cathode which themselves have a form of cylindrical symmetry, as well as some supplements according to such configuration.